Why don't the raisins in Raisin Bran all wind up at the bottom of the box?

September 14, 1990

Dear Cecil:

Consider, if you will, that classic breakfast cereal, Raisin Bran. A Raisin Bran raisin is heavier than a Raisin Bran flake. Logic dictates that heavy things ought to fall to the bottom of the box. However, when we examine a box of Raisin Bran, we find to our surprise (and delight, of course, because we love raisins) that the raisins are evenly distributed throughout! How so? Are the raisins cunningly charged with mutually repellent magnetic forces so they space themselves uniformly? Or does Kellogg's just put the raisins in last, counting on the hamhandedness of the shipping clerks to jostle them evenly through the cereal by the time it gets to your breakfast table?

— Ed, Los Angeles

Cecil replies:

An idle mind, Edward, is the devil's workshop. There's no great mystery. Bran flakes are fairly dense and they pack themselves close together in the package, preventing the raisins from moving. Kellogg's simply mixes the flakes and raisins together when filling each box and they stay that way during shipping without much internal migration.

A more interesting question along these lines, if you don't mind my saying so, is this: how come, if you've got half a jar of shelled peanuts that's been knocking around the kitchen for a while, the big pieces wind up on top and the little chunks and crumbs wind up on the bottom, contrary to expectation? Aristotle, I think, used to wonder about this. Actually there are two reasons. First, while a crumb weighs less than a big piece, the crumbs and chunks in aggregate weigh more per unit of volume. That's because the big pieces have lots of space between them and the crumbs don't.

Then we have the mechanics of sifting to think about. It's easy for the crumbs to slip down past the big pieces to the bottom of the jar, but the big pieces don't make much headway sinking into the densely packed small stuff. So it's crumbs at the bottom, nuts on top. Maybe now you have some insight into modern corporate life.